I agree that they look very similar to the untrained eye and the Quicksilver novice user (i feel most are still at this level) but beyond that the gap is rather huge in terms of triggers, customization and the overall flexibility of Quicksilver. That, of course, sometimes comes at a price of stability and robustness but if the user knows what they are doing then there is nothing like it.
I would recommend that users who have basic needs (most people i'd guess) use the commercially available products such as Alfred while those that appreciate a fast and powerful system stick with QS.
Just my 2c.